Technical Field
The present disclosure relates to enhanced affinity T cell receptors (TCRs) and, more particularly, to using agonist selection of hematopoietic progenitor cells expressing an antigen specific TCRα to generate enhanced affinity TCRs, and to uses thereof.
Description of the Related Art
TCR gene therapy is an emerging treatment approach that can overcome many of the obstacles associated with conventional T cell adoptive immunotherapy, such as the extensive time and labor required to isolate, characterize, and expand tumor antigen-specific T cell clones (Schmitt, Ragnarsson, & Greenberg, 2009, Hum. Gene Ther. 20:1240-1248). Further benefits of gene therapy include the ability to utilize defined populations of T cells capable of long-term persistence in vivo (Berger et al., 2008, J. Clin. Invest. 118:294-305; Hinrichs et al., 2009, Proc. Natl. Acad. Sci. USA 106:17469-17474). Such T cells can be transduced with genes encoding well-characterized TCRs that have a high affinity for tumor antigens, thereby increasing the likelihood of mediating an antitumor effect. Indeed, a recent report of therapy targeting advanced B cell leukemia with genetically modified T cells expressing a high affinity chimeric receptor targeting a self/tumor-antigen has highlighted the potential of using engineered high avidity T cells for the treatment of leukemia (Kalos et al., 2011, Sci. Transl. Med. 3:95ra73). However, since most tumor antigens targeted by T cell immunotherapy are over-expressed self-proteins, high affinity T cells specific for these antigens are generally subject to negative selection in the thymus. Therefore, one significant limitation of T cell based immunotherapies in general is the limited availability of T cells expressing an endogenous TCR with sufficiently high affinity for non-mutated tumor antigens.
Several strategies have been developed to enhance the affinity of TCRs intended for use in TCR gene therapy (Richman & Kranz, 2007, Biomol. Eng. 24:361-373; Udyavar et al., 2009, J. Immunol. 182:4439-4447; Zhao et al., 2007, J. Immunol. 179:5845-5854). These approaches generally entail the generation of libraries of TCR mutants that have undergone rounds of mutagenesis and subsequent screening for mutations that confer higher affinity for the target peptide/MHC ligand. Mutations are generally made in the CDR regions that are known to interact with peptide/MHC. CDR1 and CDR2 regions predominantly make contact with the MHC molecule, while the hypervariable CDR3 region primarily contacts the peptide (Wucherpfennig et al., 2010, Cold Spring Harbor Perspectives in Biology 2:a005140-a005140). Site-directed mutagenesis strategies generally target selected portions of all three of these regions, but still are not always successful in generating a higher affinity variant, and the improvements are limited to changes only in the specifically targeted regions. Moreover, mutations introduced into the MHC contact residues have the risk of potentially increasing the affinity of the TCR for MHC while decreasing the overall specificity of the receptor for its cognate peptide. Ideally, most mutations introduced to enhance the affinity of a TCR would be restricted to the CDR3 region for this reason. However, current methodologies are limited in the capacity to generate CDR3 diversity, because site-directed mutagenesis is constrained by the original length of the CDR3 region.
Given the difficulty of isolating high affinity T cells that recognize relevant tumor associated antigens, there is a continuing need for alternative methods for generating enhanced affinity TCRs.